Structural studies of E. coli polyribosomes and ribosomal subunits have shown that these macromolecues may exist in several forms. The structural differences are detected by the high-resolution technique of polyacrylamide gel electrophoresis. Amino acid starvation or treatment of cells with specific antibioticsproduces an alteration in the polyribosomes which is reflected in an increased electrophoretic mobility in the gels. In addition, under normal conditions, two forms of the 30s subunit are resolved in the gels. The primary difference between the two forms appears to reside in the presence of ribosomal protein S1 in the slower migrating form. The forms are interconvertible by addition or removal of S1. Addition of mRNA influences this conversion. It is proposed that mRNA binds to S1-containing ribosomes, forming a complex at 0 degrees in the absence of initiation factors. Additional bands ofslow and fast migrating 30s subunits have now been observed in a mutant which contains precursor 30s subunits containing precursor 16s rRNA. I propose to study the polyribosomes from antibiotic treated and amino acid starved cells by electronmicroscopy. I shall investigate the association of S1 with 30s subunits and mRNA. Finally I shall characterize the protein and rRNA composition of the precursor 30s subunit and attempt to identify the factor(s) responsible for its accumulation in the cell.